The LMDZ Offline Radar Instrument Simulator (LORIS): A versatile tool for the evaluation of cloud and precipitation in LMDZ

Abstract. The representation of clouds in Atmospheric General Circulation Models (AGCMs) remains a major challenge, with important implications for surface precipitation and the Earth radiative balance. Evaluating cloud representation in models through comparisons with remote sensing observations is essential but remains difficult. This paper presents LORIS (LMDZ Offline Radar Instrument Simulator), a new tool designed to evaluate LMDZ which is the atmospheric component of the IPSL Coupled Model. LORIS includes an advanced subgrid cloud generator, built upon COSP (CFMIP Observation Simulator Package), to represent clouds and precipitation at the subgrid scale. The radiative transfer module of LORIS (PAMTRA), enables to simulate the Doppler spectra and radar moments based on more realistic hydrometeor scattering assumptions, especially for snow particles. LORIS can be applied to a wide range of observations, including ground-based, airborne, and satellite radar measurements. Comparisons between LORIS simulations and various radar observations in polar regions demonstrate its potential for future evaluations or refinement of cloud and microphysics parameterizations included in LMDZ. Designed to be flexible and versatile, LORIS provides multiple options in its subgrid generator (e.g. cloud overlap assumptions or condensate variability), and can be adapted to different microphysics schemes. It opens new perspectives for evaluating and refining cloud and precipitation processes in LMDZ, while offering a modular framework that can readily integrate future developments of the model physics.